The rate at which new "legal highs" are flooding the drugs market in Europe continues to rise, an EU drugs agency has said, as open sale of the substances online increases their availability.

A total of 81 new psychoactive substances, also known as legal highs or designer drugs, were reported for the first time in 2013, up from 73 the previous year, the European Monitoring Centre for Drugs and Drug Addiction (EMCDDA) said.

Europe's Early Warning System, a detection mechanism described as the "first line of defence against emerging drugs", is struggling to cope with the rate at which new synthetic drugs are emerging, the EMCDDA warned.

And fears have been raised that designer drugs that have contributed to deaths are escaping detection.

Elsewhere in its European Drug Report 2013, the agency said the UK's average mortality rate due to overdose was 38.3 per million population, more than twice the average for Europe at 17 per million.

Commenting on the findings, European Commissioner for Home Affairs Cecilia Malmstrom said: "The EU Early Warning System, our first line of defence against emerging drugs, is coming under growing pressure as the number and diversity of substances continue to rise sharply."

In its European Drug report 2014, the EMCDDA said the total number of legal highs available across the continent has now soared past 350 substances.

Some 651 websites selling legal highs to Europeans were identified by the agency as it warned the open sale of the designer drugs online has increased availability to distributors and consumers.

"The technology to access these sites is increasingly being incorporated into consumer software, opening up these marketplaces to more people," the report said.

"In addition, the open sale of 'legal highs' on the Internet appears to have increased their availability to distributors and consumers."

New psychoactive substances are synthetic or naturally-occurring substances not controlled under international law, designed to mimic the effects of other drugs.

They are often mislabelled as plant food or research chemicals. Legal highs are particularly difficult to control as manufacturers, suppliers, retailers, website-hosting and payment-processing services can all be based in different countries, the EMCDDA said.

In addition, the growing use of anonymous networks, sometimes known as the "dark web", for the sale of designer drugs to dealers and consumers adds to the difficulty.

The rise of legal highs has made it increasingly complicated to assess and identify drug-induced or drug-related deaths, the EMCDDA said.

"Most overdoses occur among individuals who have consumed multiple substances, and attributing causality is often problematic," the report said.

"With the continuing release of new psychoactive substances on the drug market, there is concern that new or obscure substances that have contributed to deaths may escape detection."

It added: "The high potency of some synthetic substances further complicates their detection, as they will be present only at very low concentrations in the blood."

Among the 81 new legal highs identified in 2013, 29 were synthetic cannabinoids, substances designed to recreate the effects of cannabis. Another 30 compounds did not fit into any recognised group, the report said.

There were 13 new substituted phenethylamines reported, seven synthetic cathinones, a tryptamine and a piperazine.

Legal highs are often manufactured in China or India but there are secret laboratories producing the substances in Europe, the agency said.

The strength of designer drugs is causing difficulties for law enforcement agencies as well, as small quantities of the substances can be converted into multiple doses, the report said.

Ms Malmstrom said: "Europe's law enforcement bodies are increasingly faced with the fact that small, easily transported packages of seemingly innocuous powders can obtain many thousands of individual doses."

In the UK, the Home Office last year launched a review of legal highs to look at how the country's laws can be improved.

Options include the expansion of legislation to ensure police and law enforcement agencies have better tailored powers.

The Government banned two groups of psychoactive substances in December- NBOMe and Benzofuran compounds - as class A and B drugs respectively.

Elsewhere in the report, the EMCDDA said heroin use and availability was in decline, with the number of first-time entrants to specialist drug treatment for heroin problems falling from a peak of 59,000 in 2007 to 31,000 in 2012.

Heroin seized in 2012, five tonnes, was the lowest reported in the last decade. Drug use remains one of the major causes of mortality among young people in Europe, the report said, with around 6,100 overdose deaths reported in Europe in 2012, down from 6,500 in 2011.

Estonia had the highest average mortality rate due to overdose at 191 per million, the agency said.

Crime Prevention Minister Norman Baker said: "The coalition Government is determined to clamp down on the reckless trade in what are somewhat inaccurately called 'legal highs', which have tragically claimed the lives of far too many young people in our country.

"Hundreds of substances previously sold as 'legal highs' are controlled drugs in the UK, mainly thanks to our generic legislation.

"We currently control more than 250 new psychoactive substances in the UK, including some of the 81 substances reported for the first time by the EMCDDA.

"I have also set up an expert panel to consider how we can strengthen the UK's response and ensure those involved in breaking the law are brought to justice. The panel will report to me shortly."